My first bench was 8'x4'. 4' is really too big IMO. It was hard to reach the back of the bench. My new bench is 9'x20". 20 inches is plenty deep enough IMO. I do allow 2ft for each press. I think that gives the perfect amount of space. It's very comfortable to load on. Each one of the cabinets in the picture are 2' wide. I allow one press per cabinet.
So IMO 2' would be plenty deep. But a would go 8' wide if you have the room.